GENUS ARCELLA— ARCELLA VULGARIS. 171 



Locality. — Common in the ooze of almost all standing fresh waters, 

 and on submerged portions of aquatic plants. Nova Scotia, Maine, New 

 Jersey, Pennsylvania, Alabama, Florida, and in the Uinta Mountains and 

 valley of Fort Bridger, Wyoming Territory. 



Arcella vulgaris, represented in pi. XXVII, and figs. 1-7, pi. 

 XXVIII, is one of the most common of the shell-bearing Fresh-water 

 Rhizopods, and is found in almost every pond, ditch, or long-standing pool 

 in boggy places, creeping in the soft ooze of the bottom or in the floc- 

 culent matter adherent to submerged plants. 



As ordinarily seen (fig. 2, pi. XXVII) beneath the microscope, it 

 appears as a brown circular disk, with a paler circular central spot corre- 

 sponding with the mouth. In a side view (fig. 1 ), the outline is usually 

 low bell-shaped or hemispherical, with the basal border rounded or slightly 

 prominent and rounded. The height is about half the breadth of the shell, 

 but is often more or less, and the greatest width is at or just above the base. 

 The latter is mostly circular, convex downward at the periphery, and 

 concave centrally, so as to appear like an inverted funnel. The mouth 

 is central, circular, and situated at the top of the inverted funnel-like 

 base. The border of the mouth is entire (fig. 2), but sometimes is more 

 or less crenulated, as seen in fig 9. 



The dome of the shell may be evenly convex to the rounded or 

 slightly prominent base. Often its summit and sides are depressed into 

 a variable number of shallow concavities or more or less angular facets 

 defined by folds of the shell. The depressions are ranged into two or 

 three series, usually one on the dome and one or two at the sides, or 

 there may be a single depi-ession on the dome and one or two series at 

 the sides. 



Sometimes the shell appears like a truncated pyramid, or a tent sup- 

 ported by poles, as represented in figs. 8-13, pi. XXVIII. Such a form is 

 indicated by Ehrenberg as one of the varieties described under the name 

 of Arcella dentata, and referred to a particular species by Perty with the 

 name of A. ar,gidosa. Varieties occasionally occur with a transversely 

 oval or quadrately oval outline, with depressed summit and sides, as seen 

 in figs. 4-7 of the same plate. 



The shell of Arcella vulgaris is usually of some shade of ochre-yellow 



